This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Tetherin (BST-2 or CD317) is an interferon-inducible transmembrane protein that inhibits virus release from infected cells. Whereas HIV-1 Vpu and HIV-2 Env antagonize human tetherin, most SIVs use Nef to antagonize the tetherin proteins of their simian hosts. Here we show that compensatory changes in the cytoplasmic domain of gp41 restore resistance to tetherin in a nef-deleted strain of SIV that regained a pathogenic phenotype in rhesus macaques. These changes facilitate virus release in the presence of rhesus tetherin, but not human tetherin, and enhance virus replication in interferon-treated lymphocytes. The changes in gp41 result in a selective physical association with rhesus tetherin, and the internalization and sequestration of rhesus tetherin by a mechanism that depends on a conserved endocytosis motif in gp41. These results are consistent with HIV-2 Env antagonism of human tetherin, and suggest that the ability to oppose tetherin is important for lentiviral pathogenesis.